Appendix II. 801
increases fluidity and makes casting possible with the whiter
irons. (Seep. 1014.)
Aluminium Bronze, as a substitute for gold, has been
long known, but has only recently been advocated as an engineer's
metal. The best proportions are 5 to 11 % of aluminium, the
rest being copper; and the strengths are, with
5% Al. 24-5 tons sq. in., breaking. 40% elongation.
11% Al. 35*7 tons sq. in., breaking. 10% elongation.
If a small portion of silicon be added, the strength is increased
but the ductility diminished. The ro % alloy is much used for
bearings, gear wheels, propellers, &c. Shrinkage when casting is
very great, and good feeding gates are necessary.
Manganese Bronze has been mentioned at p. 85. The
ferro-manganese usually added is objectionable as introducing
iron, which decreases toughness and increases corrosion, so it is
better to use an alloy of manganese and copper. One of the best
and cheapest manganese bronzes has the following percentage
Copper ... ... 53
Zinc...... ... 42
Manganese ... 3*75
Aluminium ... 1*25
In view of the competition between the bronzes for propeller
construction, it may be noted that the relative cost for different
metals is given by the subjoined figures 4
Aluminium Bronze -
P. 98. Steam Hammer Blow.For the benefit of some
readers an amplification is here given of the matter on p. 98 -:.,.
Velocity due to tup weight = ^/2^-H (H = height of
. , ^ , , - Total pressure PF
Acceleration due to steam =--------£----- = -£..